Attention-deficit/hyperactivity disorder (ADHD) is a frequent diagnosis among children with emotional and behavioural difficulties. However, it is unclear which specific "beliefs" teachers hold about the disorder and how these shape their "expectations." Essentialist beliefs about ADHD (the assumption that individuals with ADHD are fundamentally different from others) are prevalent in scientific and public discourse, but they may raise inauspicious expectations that could trigger negative self-fulfilling prophecies. In an online study, we randomly assigned preservice teachers (N = 213) to a group in which essentialist beliefs about ADHD were induced vs. a control group. The experimental manipulation induced essentialist beliefs, though only in a subgroup. In participants who had more previous experience with ADHD, essentialist beliefs could be induced and were associated with some of the pessimistic expectations studied. Our preliminary findings appear promising and should be further investigated to improve teacher training and to better support children with ADHD.